This invention relates to the measurement of the electrical resistivity of particulate material entrained in gas streams, and, more particularly, to the measurement of the electrical resistivity of the entrained particulate material in power plant combustion gases that are to be subjected to electrostatic precipitation treatment.
In a coal-fired power plant, coal is burned to heat air, which in turn boils water to form steam. The steam drives a turbine and thence an electric generator, producing electricity. Besides heat, the burning of the coal produces gaseous pollutants such as sulfur and nitrogen oxides, and a solid particulate known as fly ash. Environmental protection laws mandate that the amounts of gaseous pollutants and solid particulate emitted from the power plant be maintained at acceptably low levels, and the present invention deals generally with the technology for controlling particulate emissions.
One widely used approach for removing the particulate fly ash from combustion gas streams is electrostatic precipitation. The combustion gas stream with entrained particulate is passed between highly charged electrodes that ionize the particles so that they are attracted to, and deposited upon, a collection electrode. The particulate may optionally be charged prior to entry into the precipitator to increase the efficiency of removal. The cleaned combustion gases are released to the atmosphere, and the precipitated particulate is removed from the collection electrode.
The efficiency of operation of electrostatic precipitators depends markedly upon the electrical resistivity of the particulate. If the resistivity is too high, a collection current cannot be induced to flow between the electrodes of the precipitator, so that the ability to collect particulate is reduced, sometimes severely so. There exist conditioning procedures and apparatus for altering the conductivity of the particulate by injection of conditioning agents into the combustion gas stream prior to its entering the electrostatic precipitator.
An example of such a treatment procedure is that disclosed in U.S. Pat. No. 3,993,429, and this approach has become widely accepted and used throughout the United States and the world. In this approach, a conditioning gas such as sulfur trioxide or ammonia is injected into the combustion gas stream. In the case of sulfur trioxide, the conditioning gas reacts with water in the gas stream to produce sulfuric acid that is deposited upon the surface of the particulate. The ionized sulfuric acid reduces the electrical resistance of the particulate, which in turn raises the electrical conductivity of the fly ash particulate so that the electrostatic precipitation treatment works well. Conditioning treatments are routinely used where the sulfur content of the coal burned in the power plant is so low that the electrical resistivity of the resulting particulate is too high to permit the electrostatic precipitators to operate properly.
The proper amount of conditioning gas to inject, and the best operating settings of the precipitators, must be determined to permit optimum operation of the system. To achieve operating control of the precipitator and/or the conditioning system, it is desirable to know the electrical resistivity of the particulate being precipitated on an ongoing basis, using measurements within the apparatus. With this knowledge, the operation of the system can be optimized in real time under manual or automatic control. If the resistivity of the particulate is not known, then the proper operating parameters can only be approximated, based upon the experience of the operator.
There have been developed various types of apparatus for measuring the electrical resistivity of the particulate. These include the Southern Research Institute point-to-plane probe, the Wahlco cyclonic probe, and the interlocking comb probe. All of these probes permit measurement of some resistivity parameter, but all also suffer from certain shortcomings. The objective of the measurement is to determine the resistivity of the particulate under as realistic conditions as possible, simulating the conditions within the electrostatic precipitator. None of the known types of probes collect truly representative samples of the particulate under the conditions of precipitation. Moreover, none of the known apparatus are continuous or semicontinuous in operation, and must be inserted into the flowing gas stream on an intermittent basis to accomplish the measurement.
There therefore exists a need for an improved apparatus and method for collecting samples and measuring the resistivity of particulate entrained in a gas stream, in a wide variety of circumstances. This need is particularly acute for the measurement of the resistivity of fly ash particulate in the combustion gas stream of coal-fired power plants. Such an approach desirably would collect a representative sample of particulate under precipitator operating conditions. Also, the approach should permit continuous or nearly continuous measurement of resistivity, to allow real-time, continuous control of the gas cleanup system, where present. The present invention fulfills this need, and further provides related advantages.